1. Field of the Invention
The present invention relates generally to a piezo-resonator and a filter using the resonator. More particularly, the invention relates to a piezo-resonator for use in an intermediate frequency filter, which can be provided in, for example, an amplitude modulation (AM) radio. The invention also relates to a filter and AM signal receiving circuit containing the above-described resonator.
2. Description of the Related Art
Referring to a perspective view illustrating an example of known types of piezo-resonators shown in FIG. 27, a piezo-resonator generally designated by 1 includes a rectangular piezoelectric substrate 2. Electrodes 3 and 4 are formed on both top and bottom surfaces of the piezoelectric substrate 2. One electrode 3 is split into two electrode portions 3a and 3b along the width of the substrate 2 across a groove 5, the split portions 3a and 3b being used as input/output terminals 6a and 6b, respectively. The other electrode 4 serves as a ground electrode.
In this piezo-resonator 1, the longitudinal vibration mode of a piezoelectric device can be generated by using the two electrode portions 3a and 3b as the input/output electrodes. The frequency characteristics of the piezo-resonator 1 are shown in FIGS. 28 and 29. FIG. 29 illustrates the characteristics of and around the resonant frequency which are partially enlarged from the diagram of FIG. 28. It is seen that the piezo-resonator using the longitudinal vibration mode can be made smaller than the piezo-resonators operating in a square-type vibration mode.
The piezo-resonator operating in the longitudinal vibration mode can be used in an intermediate frequency filter to be installed in, for example, an AM radio. In an AM signal receiving device, a mixing circuit mixes a received signal with a signal output from a local oscillation circuit. An output signal from the mixing circuit passes through an intermediate frequency filter to extract an intermediate frequency signal. The intermediate frequency signal is then amplified in an intermediate-frequency amplifying circuit and further detected in a detection circuit. Subsequently, the detected signal is amplified in a low-frequency amplifying circuit to output an amplified signal, which then drives a speaker.
However, any external impact applied to the radio causes warpage and deformation of the piezo-resonator as illustrated in FIG. 30. Due to this warpage and deformation, one side of the piezoelectric substrate is expanded, while the other side is contracted, thereby generating electrical charges with opposite polarities in the substrate. These charges are disadvantageously output from the input/output terminals as a signal, which is further transmitted to a radio amplifier, to finally drive a speaker, causing the occurrence of noise. Thus, once the radio is dropped or receives a force sufficient to deform the piezo-resonator, the deformed piezo-resonator continuously generates electrical charges with opposite polarities resulting in the output of undesired signals which is heard as noise.